1. Field of the Invention
This invention relates to laboratory equipment used for performing sequential chemical reactions of which the polymerase chain reaction (PCR) is an example. In particular, this invention relates to the reaction vessels used in conjunction with thermal cyclers.
2. Description of the Prior Art
PCR is one of many examples of chemical processes that require precise temperature control of reaction mixtures with rapid and precise temperature changes between different stages of the process. PCR itself is a process for amplifying DNA, i.e., producing multiple copies of a DNA sequence from a single strand bearing the sequence. PCR is typically performed in instruments that provide reagent transfer, temperature control, and optical detection in a multitude of reaction vessels such as wells, tubes, or capillaries. The process includes a sequence of steps that are temperature-sensitive, different steps being performed at different temperatures in a sequence that is repeated a multitude of times to obtain a quantity large enough for analysis and study from a starting sample of extremely small volume.
While PCR can be performed in any reaction vessel, multi-well plates are the vessels of choice. In many applications, PCR is performed in “real-time” and the reaction mixtures are repeatedly analyzed throughout the process, using the detection of light from fluorescently-tagged species in the reaction medium as a means of analysis. In other applications, DNA is withdrawn from the medium for separate amplification and analysis. Multiple-sample PCR processes in which the process is performed concurrently in a number of samples offer significant advantages, including high efficiency in treating a large number of samples simultaneously and the ability to compare and combine results from different samples for a variety of analytical, diagnostic, and research purposes. Concurrent processing is achieved by using a multi-well plate with one sample per well. The entire plate including all samples therein is simultaneously equilibrated to a common thermal environment in each step of the process. Multi-well plates are particularly useful in automated PCR procedures. Plates with 96 wells in an 8×12 array are typical, but plates with up to 1536 wells are also used.
To perform temperature cycling, the plate is placed in contact with a metal block, known in the industry as a “thermal block,” which is heated and cooled either by Peltier heating/cooling modules or by a closed-loop liquid heating/cooling system that circulates heat transfer fluid through channels machined into the block. The heating and cooling of the thermal block are typically under the control of a computer with input from the operator. The thermal block has a contour complementary to that of the plate wells to achieve full surface contact and hence intimate thermal contact and maximal heat transfer, between the block and each well.
The plate is typically of plastic formed by injection molding. Unfortunately, plastic is not a medium of high thermal conductivity and this causes the plate to present thermal resistance to heat transfer between the thermal block and the samples in each well. The plastic itself is thus a rate-limiting factor in the speed with which the temperature can be raised and lowered in the PCR process. The resistance of the plate to heat transfer can be lowered by reducing the plate thickness, but the typical injection molding process is limited in terms of how thin a plate can be formed thereby. Recognition of this limitation is found in Turner, United States Patent Application Publication No. US 2007/0059219 A1, publication date Mar. 15, 2007. The solution offered by Turner is the use of a two-stage molding process, the first stage involving injection of the resin into a mold cavity and the second involving moving the parts of the mold after it is closed to compress the resin and displace it within the closed cavity.
Plates that are formed with ultra-thin walls tend to have low rigidity, which leads to dimensional instability. In commercially viable PCR procedures, the plates and the samples that are retained in the plate wells must be manipulated by automation, and dimensional stability is required for reliable movement and positioning of the plates as well as the accurate movement of samples and reagents into and out of the individual wells. One solution is offered by Hans-Knöll Institut, European Patent Application Publication No. EP 1 000 551 A1, publication date May 17, 2005, and its counterpart, United States Patent Application Publication No. US 2004/0214315 A1, publication date Oct. 8, 2004. The plate in the Hans-Knöll Institut document is constructed with a rigid frame that surrounds the central area occupied by the wells and is joined to the central area by heat bonding. This design is of limited effect since the rigid frame occupies only the periphery of the plate, leaving the relatively large center section vulnerable to buckling. A further difficulty is that heat bonding is of limited reliability as a means of keeping the sections of the plate properly joined.